1. Field of the Invention
The present invention relates to a novel lipoprotein particle, methods for preparing and purifying the same, its use in medicine, particularly in the prevention of malarial infections, compositions/vaccines containing the protein or antibodies against the protein particle such as monoclonal or polyclonal antibodies and use of the same, particularly in therapy.
2. Description of Related Art
Malaria is one of the world's major health problems with more than 2 to 4 million people dying from the disease each year. One of the most prevalent forms of the disease is caused by the protozoan parasite P. vivax, which is found in tropical and sub-tropical regions. Interestingly the parasite can complete its mosquito cycle at temperatures as low as 15 degrees Celsius, which has allowed the disease to spread in temperate climates.
One of the most acute forms of the disease is caused by the protozoan parasite, Plasmodium falciparum (P. falciparum) which is responsible for most of the mortality attributable to malaria.
The life cycle of Plasmodium is complex, requiring two hosts, man and mosquito for completion. The infection of man is initiated by the introduction of sporozoites in the saliva of an infected mosquito. The sporozoites migrate to the liver and there infect hepatocytes where they differentiate, via the exoerythrocytic intracellular stage, into the merozoite stage which infects red blood cells (RBC) to initiate cyclical replication in the asexual blood stage. The cycle is completed by the differentiation of a number of merozoites in the RBC into sexual stage gametocytes, which are ingested by the mosquito, where they develop through a series of stages in the midgut to produce sporozoites which migrate to the salivary gland.
Due to the fact that the disease caused by P. vivax is rarely lethal, efforts to prevent and treat malaria have been focused on the more deadly form of the disease caused by Plasmodium falciparum (P. falciparum).
Although the disease caused by P. vivax does not usually result in death of the patient, due to the volume of cases, which seems to be increasing, the significant impact on the quality of life of the patient, the increasing reports of the severe incidences of the disease resulting in anemia and death, and the economic impact, an effective vaccination for the disease is still required. Furthermore, a single vaccine able to provide protection against both causes of the disease would be advantageous.
A feature of the P. vivax is that some strains are capable of causing delayed infection by remaining latent in the liver before emerging into the peripheral circulation to manifest clinical symptoms. Thus individuals, for example when traveling through an infected area, may be infected and yet may not exhibit symptoms for several months. This has the potential to cause the spread of the disease and for this reason persons traveling to infected areas are not allowed to donate blood for transfusion for a defined period of time after traveling to the infected region.
P. vivax malaria infection remains latent within the liver while the parasite is undergoing pre-erthrocytic shizogony. If the parasite is controlled at this stage, before it escapes the liver, no clinical symptoms of the disease, are observed in the patient.
The sporozoite stage of Plasmodium has been identified as a potential target of a malaria vaccine. Vaccination with deactivated (irradiated) sporozoite has been shown to induced protection against experimental human malaria (Am. J, Trop. Med. Hyg 24: 297-402, 1975). However, it is has not been possible practically and logistically to manufacture a vaccine for malaria for the general population based on this methodology, employing irradiated sporozoites.
The major surface protein of the sporozoite is known as circumsporozoite protein (CS protein). It is thought to be involved in the motility and invasion of the sporozoite during its passage from the initial site of inoculation by the mosquito into the circulation, where it migrates to the liver.
The CS protein of Plasmodia species is characterized by a central repetitive domain (repeat region) flanked by non-repetitive amino (N-terminus) and carboxy (C-terminus) fragments.
The central domain of P. vivax is composed of several blocks of a repeat unit, generally of nine tandem amino acids.
In certain Asian strains, after the central repeat region, an additional sequence of approximately 12 amino acids is present (see SEQ ID No 11). The function of the latter is not known. However, it is hypothesized, by some, that said amino acids may be linked to the delayed onset of clinical symptoms of the disease, although this has not been investigated. It is thought that the N-terminus is characterised by a sequence of 5 amino acids known as region I (see SEQ ID No 1). It is also thought that the C-terminus is characterised by comprising a sequence of 18 amino acids known as region II. The latter contains a cell-adhesive motif, which is highly conserved among all malaria CS protein (see SEQ ID No. 2).
Several groups have proposed subunit vaccines based on the circumsporozoite protein. Two of these vaccines have undergone clinical testing; one is a synthetic peptide, the other is a recombinant protein (Ballou et at Lancet: i 1277 (1987) and Herrington et at Nature 328:257 (1987)). These vaccines were successful in stimulating an anti-sporozoite response. Nonetheless, the magnitude of the response was disappointing, with some vaccinees not making a response at all. Furthermore, the absence of “boosting” of antibody levels after subsequent injections and results of in vitro lymphocyte proliferation assays suggested that T-cells of most of these volunteers did not recognise the immuno-dominant repeat. Nonetheless, one volunteer vaccinated in each study did not develop parasitemia.
WO 93/10152 and WO 98/05355 describe a vaccine derived from the CS protein of P. falciparum and it seems that there has been some progress made towards the vaccination against P. falciparum using the approach described therein, see also Heppner et al. 2005, Vaccine 23, 2243-50.
The CS protein in P. falciparum has a central repeat region that is conserved. In contrast at least two forms (designated VK210 or type I and VK247 or type II) of the CS protein for P. vivax are known. This renders it more difficult to identify a construct of the CS protein with all the desired properties such as immogenicity, which provides general protection against P. vivax regardless of the specific type of CS protein because antibodies directed the central repeating region of type I do not necessarily recognize epitopes on the corresponding region of type II and vice versa.
A recombinant P. vivax CS protein was expressed and tested as a vaccine in the 1980-1990's with limited success (Collins et al., 1989. Am. J. Trop. Med. Hyg. 40, 455-64). Some work has been done to develop a vaccine based on Multiple Antigen Peptides (MAP) employing one or more epitopes that are cross-linked (Nardelli and Tam, 1995, Pharm. Biotechnol. 6, 803-19).